Material for gaseous diffusion electrode

Abstract

A material for use as a gaseous diffusion electrode is provided. The material has improved durability resulting in long life. the material comprises a composite of a first layer of polytetrafluoroethylene (PTFE) containing a filler of a powdered electroconductive substance and a second layer of PTFE in bonded laminar contact with the first layer. The second layer preferably contains no electroconductive filler. The first and second layers are bonded together under pressure to form an intermediate composite and this intermediate composite is stretched, to produce the material of the invention which is a composite layered product of expanded PTFE having nodes interconnected by fibrils, one layer containing electroconductive material and the second layer preferably containing no electroconductive material. A catalytic substance can be incorporated onto or into at least the first layer by either deposition or by a filler of catalytic powder for use as the gas diffusing electrode of the invention.

What is claimed is: 
     
       1. A composite material for use as a gas diffusing electrode comprising: (a) a first layer comprising a sheet of expanded, porous PTFE having nodes interconnected by fibrils and having an initial thickness, said first layer containing a powdered, electroconductive substance therein, and   (b) a second layer comprising a sheet of pure, expanded, porous PTFE having nodes interconnected by fibrils and having an initial thickness,   said second layer being more highly porous than said first layer and being bonded to and integrated with said first layer, the thickness of said composite material being less than the combined initial thicknesses of said first and second layers, a portion of said nodes and fibrils of said first and second layers being interconnected and intertwined to form said bond between the layers.   
     
     
       2. The material of claim 1 wherein said first layer comprises 30% to 95% by weight of said powdered, electroconductive substance and 5% to 70% by weight of PTFE. 
     
     
       3. The material of claim 1 wherein said powdered, electroconductive substance is powdered carbon. 
     
     
       4. The material of claim 1 wherein said first layer contains a catalytic substance. 
     
     
       5. The composite material of claim 1 wherein said second layer contains a powdered, electroconductive substance therein. 
     
     
       6. The method of making a layered composite material for use as a gas diffusing electrode comprising the steps of: (a) blending together a finely powdered PTFE, a powdered electroconductive substance and a liquid lubricant to form a pasty blend,   (b) compressing, extruding and rolling said pasty blend to form a sheet of PTFE containing said substance and lubricant, to be used as a first basic layer of said composite, said first layer having an initial thickness,   (c) superimposing on said first layer a second layer comprising a sheet of pure, expanded porous PTFE, with or without lubricant and having a higher porosity than said first layer, having an initial thickness,   (d) compressing the total thickness of said superimposed layers to a thickness less than the combined initial thicknesses of said first and second layers, to form an intermediate composite,   (e) removing said liquid lubricant from said intermediate composite, and   (f) stretching said intermediate composite to form said composite for use as a gas diffusing electrode.   
     
     
       7. The process of claim 6 wherein said first layer comprises 30% to 95% by weight of said powdered, electroconductive substance and 5% to 70% by weight of PTFE. 
     
     
       8. The process of claim 6 wherein said powdered, electroconductive substance is powdered carbon. 
     
     
       9. The process of claim 6 comprising applying a catalytic substance to at least said first layer. 
     
     
       10. The process of claim 6 wherein said second layer contains a powdered, electroconductive substance therein.